In high current bridge applications, electronic switches may be used to couple a load to a high side or a low side supply line. Mostly, n-channel Power MOSFETs are used as switches in such applications, but other kinds of switches are possible as well. In order to be able to drive the gates and the gate charges of these switches very fast, so called bridge driver integrated circuits (ICs) are used. An alternating switching of the high side and low side switches is achieved by using a pulse width modulated (PWM) signal, which defines the load current at the output of the bridge with its duty cycle.
In order to connect the output (source) of a high side n-channel power MOSFET (Metal Oxide Semiconductor Transistor) very low ohmic to the positive supply line, a gate voltage has to be generated by a high side driver stage, which lies above the positive supply voltage of the bridge. Two different methods are known, to generate this required high side driver supply.
The first method is the so called bootstrap method, where a bootstrap capacitance, which is clocked by a PWM signal, is charged during the low side phase of the bridge, low side phase meaning that the low side switch is conducting while the high side switch is not conducting. During the high side phase it then provides the gate charge which is necessary to allow an operation of the transistor. High side phase means, that the high side switch is conducting while the low side switch is not conducting. As the bootstrap capacity is only charged during the low side phase, the high side voltage cannot be maintained, if the duty cycle gets too high. Too high in this context means about 95% and more.
The second method is the charge pump method. Here, a buffer capacitance is constantly recharged with a charge pump frequency, which is independent of the PWM frequency which is used to clock the bridge transistors. Compared to the bootstrap method, the charge pump solution is more complex and therefore more costly to implement.
Because of the disadvantages of the charge pump method, the bootstrap method is used more often, even if the duty cycle is limited to below about 95%. The charge pump method, in the majority of cases, is only used, to provide a conservation charge to the bootstrap capacity, to compensate the bias and leakage currents to permanently keep the high side gate voltage (100% duty cycle).
A solution is needed, which allows duty cycles over the full range of 0 to 100%, without being too costly and complex to implement.